The present invention relates to a method for forming a refractory metal silicide layer, and more particularly to a method for forming a refractory metal silicide layer self-aligned in a scaled down MOS field effect transistor.
In recent years, the requirement for scaling down in vertical and lateral dimensions of semiconductor devices for realization of the high density integration has been on the increase. Sub-quarter micron size memory devices and logic devices, for example, in the range of 0.15-0.25 micrometers have been now required for ultra large scale integrated circuits. Such high density integration of the semiconductor devices requires lateral and vertical size scaling down of the semiconductor elements such as MOS field effect transistor. Reductions in gate length and source/drain diffusion layer width are required as a lateral scaling down as well as reduction in thickness of the layers provided in the semiconductor element is also required as a vertical scaling down. Such reductions in gate length and source/drain diffusion layer width as well as reduction in thickness of the layers in the semiconductor elements, however, result in increase of in resistance thereof whereby the issue of circuit delay due to the increased resistance thereof is raised. Reduction in resistance of the semiconductor device is an essential issue for obtaining high speed performances of the ultra large scale integrated circuits. It is preferable to utilize a metal silicide layer for reduction in resistance of the semiconductor device and for scaling down thereof. Particularly, a refractory metal silicide such as titanium silicide is more preferable. In order to selectively form a fine refractory metal silicide layer on a limited small area, there had been used a self-aligned silicide (salicide) technique. Particularly, this salicide technique is important for scaling down of the MOS field effect transistor with high speed performance.
In the light of the scaling down of the MOS transistor with high speed performance, it is preferable that the source/drain diffusion regions are shallow. The silicidation reaction of refractory metal with silicon appears in the surface of the source/drain diffusion regions underlying the refractory metal layer. Namely, the refractory metal silicide layer is formed in upper regions of the source/drain diffusion layers. If the junction depth of the source/drain diffusion regions is shallower than the depth of the bottom of the refractory metal silicide layer, this means entire parts of the source/drain diffusion layers are occupied and replaced by the refractory metal silicide layer whereby the silicide layer is made into contact with the silicon substrate. This causes a crystal defect which may cause a leakage of current. In order to prevent this, it is required that the refractory metal silicide layer be formed within upper region of the source/drain diffusion regions or shallower than the source/drain diffusion regions.
Accordingly, in order to realize a substantial scaling down of the semiconductor device such as MOS field effect transistors including refractory metal silicide layers, it is essential to form the refractory metal silicide layer extremely shallow or form an extremely thin refractory metal silicide layer.
One of the conventional methods for forming the MOS field effect transistor by use of the salicide technique is disclosed in the Japanese patent publication No. 3-65658 and will be described as follows.
With reference to FIG. 1A, a silicon substrate 101 was prepared. Field oxide films 102 were selectively formed by a local oxidation of silicon method on a surface of the silicon substrate 101 so as to define an active region surrounded by the field oxide films 102. Further, an ion implantation of impurity into the active region was carried out to form channel stoppers. Subsequently, a thermal oxidation of silicon was carried out to form a gate oxide film 103. A chemical vapor deposition method was carried out to form a polysilicon film having a thickness of 150 nanometers over an entire region of the substrate 101. An n-type impurity such as phosphorus was doped into the deposited polysilicon film so as to reduce a resistivity of the polysilicon film. The impurity doped polysilicon film was then patterned by a photolithography to form a gate electrode 104. A chemical vapor deposition was carried out to deposit a silicon oxide film on an entire region of the substrate 101. The deposited silicon oxide film was then subjected to an anisotropic etching to form side wall oxide films 105 at opposite sides of the gate electrode 104. An ion-implantation of arsenic into the substrate 101 was carried out and then the substrate 101 was subjected to a heat treatment at a temperature of about 800-1000.degree. C. to cause a diffusion of the doped impurity whereby source/drain regions 106 were formed.
With reference to FIG. 1B, a titanium film 107 having a thickness of about 50 nanometers was deposited by a sputtering method on an entire region of the substrate 101.
With reference to FIG. 1C, the substrate 101 was subjected to a lamp anneal in a nitrogen atmosphere at a temperature of 600-650.degree. C. for 30-60 seconds whereby a silicidation reaction of titanium with silicon appeared on interfaces of the titanium film 107 to the polysilicon gate electrode 104 and to the silicon diffusion layers 106 acting as the source/drain regions. As a result, C49 structured titanium silicide layers 109 having a resistivity of about 60 .mu..OMEGA.cm were formed on the interfaces of the titanium film 107 to the polysilicon gate electrode 104 and to the silicon diffusion layers 106.
With reference to FIG. 1D, the nitrogen containing titanium film 110 was removed by a wet etching which uses a chemical in which hydrogen peroxide is mixed in an ammonia solution so as to leave only the C49 structured titanium silicide layer 109 over the polysilicon gate electrode 104 and over the source and drain diffusion regions 106.
With reference to FIG. 1E, the substrate 101 was subjected to a secondary heat treatment in an argon atmosphere at a temperature of 850.degree. C. for 60 seconds to cause a phase transition of the C49 structured titanium silicide layer 109 into a C54 structured titanium silicide layer 111 having a resistivity of 20 .mu..OMEGA.cm. The C54 structured titanium silicide layer 111 has a lower resistivity than the C49 structured titanium silicide layer 109, for which reason a sheet resistance of the titanium silicide layer is reduced by the secondary heat treatment.
Another of the conventional methods for forming the MOS field effect transistor by use of the salicide technique is disclosed in the Japanese patent publication No. 3-73533 and will be described as follows.
With reference to FIG. 2A, a silicon substrate 101 was prepared. Field oxide films 102 were selectively formed by a local oxidation of silicon method on a surface of the silicon substrate 101 so as to define an active region surrounded by the field oxide films 102. Further, an ion implantation of impurity into the active region was carried out to form a channel stopper. Subsequently, a thermal oxidation of silicon was carried out to form a gate oxide film 103. A chemical vapor deposition method was carried out to form a polysilicon film having a thickness of 150 nanometers over an entire region of the substrate 101. An n-type impurity such as phosphorus was doped into the deposited polysilicon film so as to reduce a resistivity of the polysilicon film. The impurity doped polysilicon film was then patterned by a photolithography to form a gate electrode 104. A chemical vapor deposition was carried out to deposit a silicon oxide film on an entire region of the substrate 101. The deposited silicon oxide film was then subjected to an anisotropic etching to form side wall oxide films 105 at opposite sides of the gate electrode 104. An ion-implantation of arsenic into the substrate 101 was carried out and then the substrate 101 was subjected to a heat treatment at a temperature of about 800-1000.degree. C. to cause a diffusion of the doped impurity whereby source/drain regions 106 were formed.
With reference to FIG. 2B, a titanium film 107 having a thickness of more than 20 nanometers was deposited on an entire region of the substrate 101 by a sputtering method in an argon gas. A titanium nitride film 108C is formed over the titanium film 107 by a sputtering method in an argon gas.
With reference to FIG. 2C, the substrate 101 was subjected to a lamp anneal in a nitrogen atmosphere whereby a silicidation reaction of titanium with silicon appeared on interfaces of the titanium film 107 to the polysilicon gate electrode 104 and to the silicon diffusion layers 106 acting as the source/drain regions. As a result, C49 structured titanium silicide layers 109 having a resistivity of about 15 .mu..OMEGA.cm were formed on the interfaces of the titanium film 107 to the polysilicon gate electrode 104 and to the silicon diffusion layers 106. Since the titanium film 107 is covered by the titanium nitride film 108 and the heat treatment was carried out in the nitrogen atmosphere, the titanium film 107 is not exposed to oxygen during the heat treatment for titanium silicidation, for which reason no titanium oxide film is formed. This allows a reduction in sheet resistance of the titanium silicide layer.
With reference to FIG. 2D, the nitrogen containing titanium film 110 and the titanium nitride film 108 were removed by a wet etching which uses a chemical in which hydrogen peroxide is mixed in an ammonia solution so as to leave only the C49 structured titanium silicide layer 109 over the polysilicon gate electrode 104 and over the source and drain diffusion regions 106.
With reference to FIG. 2E, the substrate 101 was subjected to a secondary heat treatment to cause a phase transition of the C49 structured titanium silicide layer 109 into a C54 structured titanium silicide layer 111. The C54 structured titanium silicide layer 111 has a lower resistivity than the C49 structured titanium silicide layer 109, for which reason a sheet resistance of the titanium silicide layer is reduced by the secondary heat treatment.
In the above two conventional methods for forming the titanium silicide layer, the heat treatment such as the lamp anneal was carried out in the nitrogen atmosphere in order to suppress over growth of the silicide layer and to use the salicide technique to advantage. During the above heat treatment, for example, the lamp anneal, not only the nitrogen atoms in the titanium nitride film 108 but also silicon atoms in the source/drain diffusion regions 106 and the polysilicon gate electrode 104 are diffused Silicon atoms are vertically and laterally diffused from the source/drain diffusion regions 106 and some of the silicon atoms move toward the titanium film 107 extending over the field oxide film 102. The diffusion of the nitrogen atoms from the titanium nitride film 108 into the titanium film 107 is, however, faster than and prior to the diffusion of the silicon. The above diffusion of the nitrogen atoms forms a nitrogen containing titanium film 110 under the titanium nitride film 108 thereby promoting a nitriding reaction of titanium with nitrogen. This nitriding reaction appears over the field oxide film and is superior over any silicidation reaction of the over-diffused silicon atoms with the titanium atoms in the titanium film 107 positioned over the field oxide film 102. For that reason, no titanium silicidation reaction appears over the field oxide film 102. Namely, over growth of the titanium silicide layer is suppressed.
If, in accordance with the above conventional methods, the heat treatment is carried out in the presence of nitrogen or in the nitrogen containing atmosphere to supply nitrogen into the refractory metal nitride layer, then a supply of nitrogen atoms into the refractory metal nitride film from the atmosphere may prevent any reduction in concentration of nitrogen existing in the titanium nitride layer during the heat treatment. The heat treatment in the presence of nitrogen further promotes and allows an excess amount of nitrogen atoms to diffuse and approach in the vicinity of the interface of the titanium layer to the source/drain diffusion layers by the heat treatment. Since the bonding reaction of nitrogen with silicon does take precedence over silicidation reaction between the refractory metal and silicon, a certain amount of the nitrogen atoms having been diffused and approached in the vicinity of the interface between the titanium layer and the source/drain diffusion layers does prevent the intended silicidation reaction of refractory metal atoms with silicon atoms. This phenomenon in question will become remarkable as the thickness of the first layer is thin. This means it difficult to do a substantial scaling down of the device.
In order to settle the above problem with over growth of the silicide layer, it had been proposed to form a metal nitride layer over the metal layer wherein the metal nitride layer contains an excess amount of nitrogen to cause a sufficient nitrogen diffusion for suppressing any over growth of the silicide layer. This technique is disclosed in the U.S. Pat. No. 5,550,084 filed on Jan. 17, 1995. Since the metal nitride layer contains such excess amount of nitrogen, there is no need to further supply nitrogen from atmosphere in the heat treatment for silicidation. Excess diffusion of nitrogen into the titanium layer may, however, suppress not only unintended silicidation over the field oxide film but also the intended silicidation over the source/drain diffusion regions and over the polysilicon gate electrode. For this reason, it is required to precisely control the amount of diffused nitrogen and a nitrogen diffusion depth. Dependence upon the nitrogen-supply from the atmosphere in the heat treatment to ensure the sufficient amount of nitrogen for suppressing the over growth of the silicide layer makes it difficult to do the required precise control of the amount of diff-used nitrogen and diffusion depth. For which reason, in accordance with the above U.S. patent, a sufficiently large amount of nitrogen had previously been introduced into the metal layer before the heat treatment is carried out in the absence of nitrogen for allowing a necessary nitrogen diffusion into the metal layer.
Whereas the above described U.S. patent is suitable for suppressing the over growth of the silicide layer, the same is, however, unsuitable in the light of difficulty to reduce thickness of the metal or refractory metal layer for scaling down of the semiconductor device. The above conventional method disclosed in the above U.S. patent utilizes the excess amount of nitrogen contained in the titanium nitride layer for suppressing the over growth of the silicide layer. This, however, means that the nitrogen diffusion rate is higher and the nitrogen diffusion depth is deeper. This further means that if the titanium nitride layer contains excess amount of nitrogen, it is difficult to have the nitrogen diff-used at a low diffusion rate and diffused shallower. This makes it difficult to reduce the thickness of the metal or refractory metal layer into which the nitrogen is diffused from the nitrogen containing metal layer. Namely, if the metal layer or the refractory metal layer has a reduced thickness for allowing and promoting the required scaling down of the semiconductor device and if the nitrogen containing metal or refractory metal layer overlying the refractory metal layer contains excess amount of nitrogen as described in the above U.S. patent, then nitrogen may diffuse and approach in the vicinity of the interface of the refractory metal layer and silicon surface by the heat treatment for the purpose of silicidation. Since the excess amount of nitrogen is contained in the nitrogen containing refractory metal layer, the nitrogen diffusion rate is high and the nitrogen diffusion depth is deep, for which reason a certain amount of the nitrogen atoms have been diffused and approached in the vicinity of the interface between the titanium layer and the source/drain diffusion layers although the intended silicidation reaction of refractory metal atoms with silicon atoms is not yet initiated. As a result, no silicidation reaction will appear by the heat treatment by the excess diffusion of nitrogen. This phenomenon in question becomes remarkable as the thickness of the first layer is reduced. This means it is difficult to do a substantial scaling down of the device as long as the conventional technique as descried in the above U.S. patent.
In addition, the prior art disclosed in the above described U.S. patent has a further disadvantage. If the silicidation reaction appears on a region surrounded by an insulation film, a silicide layer is likely to be immersed into the silicon surface due to a silicon diffusion caused by the silicidation reaction. This immersion of the silicide layer into the silicon surface causes a deformation of the refractory metal layer and the nitrogen containing refractory metal layer. Structural strengths of the both layers affect the immersion of the silicide layer into the silicon surface. High structural strengths of the refractory metal layer and the nitrogen containing refractory metal layer do suppress deformation of those layers caused by the silicidation reaction. Thus, the high structural strengths of the refractory metal layer and the nitrogen containing refractory metal layer suppress the silicidation reaction. By contrast, low structural strengths of the refractory metal layer and the nitrogen containing refractory metal layer do not suppress deformations of those layers caused by the silicidation reaction. Thus, the low structural strengths of the refractory metal layer and the nitrogen containing refractory metal layer allows the silicidation reaction. By the way, the structural strength of the refractory metal nitride layer or the nitrogen containing refractory metal layer depends upon the contents of nitrogen or the nitrogen compositional ratio to the refractory metal. If the nitrogen compositional ratio to the refractory metal is high as disclosed in the above U.S. patent, then the structural strength of the refractory metal nitride layer is also high whereby the silicidation reaction is suppressed. If, however, contrary to the above U.S. patent, the nitrogen compositional ratio to the refractory metal were low, the structural strength of the refractory metal nitride layer is also low whereby the silicidation reaction is not suppressed.
Further, the prior art disclosed in the above described U.S. patent has a further more disadvantage in the use of a single heat treatment cycle at a high temperature of not less 800.degree. C. for causing silicidation reaction and subsequent phase transition C49 structure into C54 structure. As the temperature of the heat treatment is risen up, the nitrogen diffusion rate is increased thereby the nitrogen diffusion depth becomes deep. This makes it difficult to do shallow diffusion of the nitrogen and if the refractory metal is thin for the purpose of the scaling down of the semiconductor device, then it is difficult to cause silicidation reaction. If the reduction in the thickness of the refractory metal layer is required for the scaling down of the semiconductor device, it is difficult to apply the above conventional technique for forming the nitrogen rich refractory metal layer over the refractory metal layer.
The above described conventional methods are, however, not suitable for the practical and actual requirement for sufficient reproducibility and reduction in thickness of the refractory metal layer for the purpose of scaling down of the semiconductor integrated circuit device.